Commutator motor

ABSTRACT

A commutator motor includes a stator and a rotor, and the stator includes a stator yoke and a field magnet. The field magnet is fixed onto the inner peripheral surface of the substantially cylindrical stator yoke. The stator yoke is configured by coaxially stacking a plurality of plate-like annular bodies, and the annular bodies are fixedly connected to each other by caulking at a caulking portion disposed on the annular body. In manufacture, the length of the stator yoke in the axial direction can be arbitrarily changed with the thickness of the annular body in the axial direction as a minimum unit. In case of manufacturing various stator yokes that are different in the axial length, the necessity of replacing a manufacturing machine is eliminated, thereby providing a commutator motor having the stator yoke which is low in the manufacturing costs.

BACKGROUND OF THE INVENTION

The present invention relates to a commutator motor, and moreparticularly, to a commutator motor which is employed for a power toolor the like and mainly driven by a battery, and is relatively large insize.

As a motor employed for the power tool or the like, a commutator motoris known, and the commutator motor of this type is generally driven by abattery.

Japanese Patent Application Publication No. Hei-11-136883 discloses acommutator motor which is relatively small in size. The commutator motorincludes a housing, a stator, an armature, a brush and a commutator, andthe stator includes a cylindrical stator yoke which is substantiallyrectangular columnar in contour and a field magnet. The inner peripheralsurface of the stator yoke is circular in a section vertically takenalong an axial direction thereof, while an outer peripheral surface ofthe stator yoke has a rectangular cross-section. The rectangular outersurface portion has four corners, and a yoke section projecting radiallyoutwardly is provided at each corner. A field magnet is disposed betweenthe neighboring two yoke sections. Therefore, there are four fieldmagnets in total which are fixed to positions that are radially outsideof the stator yoke and opposite to each other in the diameter directionof the stator yoke. The stator yoke is structured by stacking steelplates on each other, and the field magnet is formed of a permanentmagnet. Four field magnetic poles are generated at the stator by thefield magnet, and a magnetic field is developed by the four fieldmagnetic poles.

The brush is fixed to the housing through a brush retaining device. Thehousing is substantially cylindrical and coaxially connected to thestator yoke in an immovable manner. The brush retaining device isdisposed on and protrudes radially inwardly from an inner peripheralsurface of the housing, and the brush projects inwardly in the radialdirection of the housing by the brush retaining device. The brush iselectrically connected to a battery that constitutes a power source.

The armature is disposed inside of the stator and includes a shaft, acore and a coil. The shaft is disposed at the position of the axialcenter of the stator yoke so as to be rotatable with respect to thestator yoke. The core is fixed to the shaft and has a plurality of slotsformed therein. A conductor wire is wound around the core while beinghooked by the slots, and the wounded conductor wire forms a coil. Also,the substantially columnar commutator is coaxially fixed to the shaft ata position on the shaft opposing the brush, and the shaft is sostructured as to rotate together with the commutator and the core. Thecommutator is electrically connected to the coil and always comes incontact with the brush. An electrical current is supplied to the coilthrough the brush and the commutator so that a rotational torque isdeveloped in the armature.

In the conventional commutator motor described above, the stator yoke isstructured by stacking the steel plates on each other. However, there isno disclosure of a method for coupling the adjacent steel plates to eachother in the stacking direction. For example, it is conceivable that theadjacent steel plates are fixed to each other by caulking. However,because the conventional permanent magnet commutator motor is small insize, there is a fear that the steel plates are deformed when concavesand convexes are formed on the steel plates for caulking, and it isactually impossible to fix the steel plates by caulking. Also, becausethe field magnet is disposed on the radially outer side of the statoryoke, the magnetic flux cannot be effectively utilized, which lowersperformance of the motor.

SUMMARY OF THE INVENTION

Under the above circumstances, it is an object of the present inventionto provide a commutator motor which can effectively utilize the magneticflux in which steel plates adjacent to each other in the stackingdirection are fixed to each other by caulking so as to be coupled toeach other to provide a stator yoke.

This and other objects of the present invention will be attained by acommutator motor including an improved stator, and an armature. Thestator includes a stator yoke and a field magnet. The stator yoke has atubular shape and extends in its axial direction and has an innerperipheral surface. The field magnet is fixed to the inner peripheralsurface of the stator yoke for providing a field magnetic pole in thestator. The armature is rotatably disposed within the stator. The statoryoke is constituted by a plurality of plate-like annular bodies havingiron parts which are stacked on each other coaxially in the axialdirection of the stator yoke. Alternatively, the stator yoke isconstituted by a plurality of substantially identically configured ironplate-like arcuate bodies disposed at such position as to form a part ofvirtual annular bodies and which are stacked on each other coaxially inthe axial direction of the stator yoke. The plurality of plate-likeannular bodies or the plate-like arcuate bodies adjacent to each otherin the stacked direction are fixedly connected to each other bycaulking.

In the commutator motor of the present invention, because a field magnetis disposed on the inner peripheral surface of the stator yoke, themagnetic flux can be effectively utilized, to thereby provide ahigh-performance commutator motor. Also, because a plurality of annularbodies or arcuate bodies that are adjacent to each other in the stackingdirection are fixedly connected to each other by caulking, it ispossible that the stator yoke can be readily manufactured by stackingthe plurality of annular bodies on each other.

Also, because a plurality of plate-like annular bodies or arcuate bodiesare coaxially stacked in the axial direction of the stator yoke tostructure the stator yoke, the thickness of the annular body or arcuatebody in the axial direction can be set to a minimum unit, and the numberof stacked annular bodies or arcuate bodies is set to a desired number,thereby being capable of arbitrarily setting the length of the statoryoke in the axial direction. Since the number of stacked annular bodiesor arcuate bodies can be automatically set in a pressing machine, thesetting of the axial length can be easily changed without the necessityof replacing the machine even if the various stator yokes having axiallengths different from each other are to be manufactured. Accordingly,in manufacturing the stator yokes different in the axial length, thecosts can be reduced. Also, because the annular bodies or arcuate bodiesare manufactured by the pressing machine, the configuration of theannular bodies or arcuate bodies can be made an arbitrary shape bychanging the die of the pressing machine. For example, it is easy topartially dispose convex portions or the like on the inner peripheralsurface of the annular bodies.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a cross-sectional view showing an essential portion of acommutator motor according to a first embodiment of the presentinvention;

FIG. 2 is a cross-sectional view taken along a line II—II of FIG. 1;

FIG. 3 is a cross-sectional view showing a stator of the commutatormotor according to the first embodiment of the present invention;

FIG. 4 is a side view showing an essential portion in a state whereannular bodies are stacked on each other in a stator yoke of thecommutator motor according to the first embodiment of the presentinvention;

FIG. 5 is a cross-sectional view showing a stator of a commutator motoraccording to a second embodiment of the present invention;

FIG. 6 is a cross-sectional view showing a stator in a commutator motoraccording to a third embodiment of the present invention;

FIG. 7 is a conceptual view showing a flow of a main magnetic flux and aflow of an armature reaction magnetic flux in the commutator motoraccording to the first embodiment of the present invention;

FIG. 8 is a cross-sectional view showing a stator of a commutator motoraccording to a fourth embodiment of the present invention;

FIG. 9 is a cross-sectional view showing a stator of a commutator motoraccording to a modification to the first embodiment of the presentinvention;

FIG. 10 is a cross-sectional view showing a stator of a commutator motoraccording to a first modification to the second embodiment of thepresent invention;

FIG. 11 is a cross-sectional view showing a stator of a commutator motoraccording to a second modification to the second embodiment of thepresent invention;

FIG. 12 is a cross-sectional view showing a stator of a commutator motoraccording to a third modification to the second embodiment of thepresent invention;

FIG. 13 is a cross-sectional view showing a stator of a commutator motoraccording to a fourth modification to the second embodiment of thepresent invention;

FIG. 14 is a cross-sectional view showing a stator of a commutator motoraccording to a fifth modification to the second embodiment of thepresent invention;

FIG. 15 is a cross-sectional view showing a stator of a commutator motoraccording to a sixth modification to the second embodiment of thepresent invention; and

FIG. 16 is a cross-sectional view showing a stator of a commutator motoraccording to a modification to the fourth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A commutator motor 1 according to a first embodiment of the presentinvention will be described with reference to FIGS. 1 to 4. Thecommutator motor 1 includes a housing 1A, a stator 10, an armature 20, abrush 13 and a commutator 24 as shown in FIG. 1. The housing 1A issubstantially cylindrical, and a part of the inner periphery of thehousing 1A is fitted with an outer peripheral surface of a stator yoke11 described later. The stator 10 includes the stator yoke 11 and twofield magnets 12. The stator yoke 11 is substantially cylindrical. Thethickness of the stator yoke 11 in the radial direction is in a range offrom about 3 mm to 10 mm. The outer diameter of the stator yoke 11coincides with the inner diameter of the housing 1A and the outerperipheral surface of the stator yoke 11 is fixed to a part of the innerperipheral surface of the housing 1A.

Each of the two field magnets 12 is formed of a permanent magnet, andobtained by bending the rectangular plate-like permanent magnet intoarcuate shape. The arcuate surface of the field magnet 12 coincides withthe arc of the inner peripheral surface of the stator yoke 11, and thetwo field magnets 12 are fixed to parts of the inner peripheral surfaceof the stator yoke 11 at positions opposite to each other in thediameter direction of the stator yoke 11 by an adhesive, respectively.Accordingly, when viewed in cross-section of the housing 1A in theradial direction, as shown in FIG. 2, the field magnet 12, the statoryoke 11 and the housing 1A are arranged in the stated order from theinterior of the housing 1A toward the exterior thereof in the radialdirection. Two field magnetic poles are generated in the stator 10 bythe two field magnets 12, and a magnetic field is generated by the twofield magnetic poles.

A brush holder 13A for retaining the two brushes 13 is disposed at aportion of the inner peripheral surface of the housing 1A where thestator yoke 11 is not disposed. The brush 13 is contained and retainedwithin the brush holder 13A fixed on the inner peripheral surface of thehousing 1A and projects inwardly in the radial direction of the statoryoke 11 in a state where the brush 13 is urged by a spring (not shown)which is disposed within the brush holder 13A. The brush 13 iselectrically connected to a battery (not shown) which constitutes apower source via a lead wire (not shown). One end 11A of the housing 1Aforms a cap portion 11B that covers the one end 11A, and the other endof the housing 1A (not shown) is equipped with a fixing portion 16 thatfixes a bearing 15 for rotatably supporting a shaft 21 that will bedescribed later.

The armature 20 is disposed inside of the stator 10. The armature 20includes the shaft 21, a core 22 and a coil 23. One end 21A of the shaft21 is supported by a bearing 14 disposed in the cap portion 11B, and theother end of the shaft 21 is supported by the bearing 15 disposed in thefixing portion 16. The rotation axis of the shaft 21 is coincide withthe axial center of the stator yoke 11. The core 22 is fixed to a partof the shaft 21, and a plurality of slots (not shown) are formed in thecore 22. A conductor wire is wound around the core 22 so as to be hookedby the slots (not shown), and the wounded conductor wire forms the coil23. The coil 23 faces the field magnet 12 of the stator 10.

The substantially columnar commutator 24 is coaxially fitted to theshaft 21 at a portion close to one end 21A of the shaft 21, and thecommutator 24 is rotatable together with the shaft 21 and the core 22.The commutator 24 is electrically connected to the coil 23. Also, thecommutator 24 is so positioned in contact with the brush 13. Electricalcurrent is supplied to the coil 23 from the battery (not shown) throughthe brush 13 and the commutator 24 so as to generate a rotational torquein the armature 20. Also, a cooling fan 17 is fitted to the shaft 21,and the cooling fan 17 is so structured as to rotate together with theshaft 21 to cool the armature 20.

Next, the stator yoke 11 will be described in more detail. The statoryoke 11 is structured by annular bodies 11C as shown in FIG. 3. Theannular bodies 11C are made of iron and disposed in plural numbers, andas shown in FIG. 4, the annular bodies 11C are coaxially stacked on eachother in the axial direction of the stator yoke 11. The annular bodies11C are manufactured by punching a plate-like iron plate by aniron-plate press die. For that reason, the annular bodies 11C has asmall thickness in the axial direction of the stator yoke 11. The orderin which the annular bodies 11C are stacked corresponds to theprogressive press.

Four caulking portions 11D are disposed at positions between the outerperiphery and the inner periphery of the annular bodies 11C,respectively, and the respective two caulking portions 11D are disposedalong the peripheral direction at positions corresponding to the fixingposition to which two field magnets are fixed, respectively, after theannular bodies 11C are stacked on each other. The respective twocaulking portions 11D have a rotationally symmetric positionalrelationship about the axial center of the stator yoke 11, respectively,and the positions at which the caulking portions 11D are disposed are inthe vicinity of the center position of the field magnetic pole in thestator 10.

Regarding neighboring two annular bodies, one annular body has a firstcontact face facing a second contact face of the other annular body, andthe caulking portions 11D at the first contact face of one annular bodyform convex portions. On the other hand, caulking portions 11D at thesecond contact face of the other annular body form concave portionsfittable with the convex portions. Therefore, in the plurality ofannular bodies 11C that are coaxially stacked on each other in the axialdirection of the stator yoke 11, the convex portions of the caulkingportions 11D of one annular body 11C is fitted into the concave portionsof the caulking portions 11D of the other annular body 11C that isadjacent to the one annular body 11C, respectively, to thereby performcaulking fixing. After the annular bodies 11C are stacked on each otherand then fixed by caulking to manufacture a cylindrical stator yoke 11,the field magnet 12 is fixed onto the inner peripheral surface of thestator yoke 11 by an adhesive.

Because the annular bodies 11C are fixed by caulking through thecaulking portions 11D, it is possible that the plurality of annularbodies 11C are readily stacked on each other to manufacture the statoryoke 11. Also, because the field magnet 12 is fixed onto the innerperipheral surface of the substantially cylindrical stator yoke 11, theloss of a magnetic force can be reduced so that a high-performancecommutator motor 1 can be provided. Also, since the width of therespective annular bodies in the radial direction is 3 mm to 10 mm,which is relatively large, the caulking portions can be readily providedby press machining.

Also, because the stator yoke 11 is formed by stacking the plate-likeannular bodies 11C, the thickness of the annular body 11C in the axialdirection can be the minimum unit length. When the number of stackedannular bodies 11C is set to a desired number, the length of the statoryoke 11 in the axial direction can be arbitrarily set. Also, since thenumber of stacked annular bodies 11C can be automatically set in a pressmachine, it is possible that the setting of the axial length can bereadily changed without the necessity of replacing the machine even ifvarious stator yokes 11 having the axial lengths different from eachother are to be manufactured. Accordingly, cost reduction can beachieved in manufacturing various stator yokes 11 having axial lengthsdifferent from each other.

A commutator motor according to a second embodiment of the presentinvention will next be described with reference to FIG. 5. Thecommutator motor according to the second embodiment is different fromthe commutator motor 1 according to the first embodiment only in thatthe field magnet 12 is retained on the inner peripheral surface of thestator yoke 11 not by an adhesive but by convex portions 31E protrudingradially inwardly from the inner peripheral surface of the stator yoke11.

As shown in FIG. 5, the annular body 31C that constitutes a part of thestator yoke 31 which retains the field magnets 12 on the innerperipheral surface thereof is equipped with two pairs of convex portions31E, i.e., four in total, for retaining the two field magnets 12. Theconvex portions 31E are disposed at positions of the end portions of thefield magnets 12, which are retained on the inner peripheral surface ofthe stator yoke 31, respectively, in the peripheral direction of thestator yoke 31. The convex portions 31E project inwardly in the radialdirection of the stator yoke 31, and the thickness of the stator yoke 31in the radial direction at the portions where the convex portions 31Eare disposed becomes larger than that of remaining portions. A distancebetween the respective pairs of convex portions 31E in the peripheraldirection of the stator yoke 31 is slightly smaller than the length ofthe field magnet 12 in the same direction, and one of the field magnets12 is press-inserted between one pair of convex portions 31E so as to beheld between the pair of convex portions 31E. Similarly, the other fieldmagnet 12 is press-inserted between the other pair of convex portions31E so as to be held between the other pair of convex portions 31E.Therefore, no adhesive is required for fixing the field magnets 12 onthe inner peripheral surface of the stator yoke 31.

The convex portions 31E are not attached to the manufactured annularbodies later, but are manufactured integrally with the annular body bythe press machine. The convex portions 31E are linearly arrayed in theaxial direction of the stator yoke 3 on the inner peripheral surfacethereof.

Because the field magnet 12 can be retained by the convex portions 31Edisposed on the annular body 31C, it is unnecessary to conductpositioning of the field magnet 12 in mounting the field magnets 12 onthe inner peripheral surface of the stator yoke 31. Also, because thefield magnet 12 is retained on the inner peripheral surface of thestator yoke 31 by holding the field magnet 12 between a pair of convexportions 31E, the field magnet 12 can be fixed on the inner peripheralsurface of the stator yoke 31 without using an adhesive. In the case ofusing the adhesive, there is required a jig for holding the field magnet12 in press contact with the stator yoke 31 while the adhesive isapplied and dried. Also, a time is consumed for drying the adhesive, andthe manufacturing costs are increased. However, because the adhesive isnot used in this embodiment, the manufacturing costs can be reduced.

Incidentally, in the conventional commutator motor, the stator yoke ismanufactured by bending machining, and when the thickness of the statoryoke in the radial direction is 3 mm or less in this case, the yokeportion of the stator yoke is notched and bent, thereby being capable ofproviding the convex portions on the inner peripheral surface of thestator yoke. However, in this case, since the notched portion becomes acavity, so that the yoke portion of the stator yoke does not effectivelyserve as a magnetic path, a main magnetic flux generated by the fieldmagnet is reduced, and the efficiency of the commutator motor islowered. Alternatively, an external force is exerted on the yoke portionof the stator yoke from the outer periphery to plastically deform thestator yoke so that a projection can be provided on the inner surface.However, since the yoke portion of the deformed stator yoke is reducedin thickness in the radial direction of the stator yoke due to a tensionor a compression applied during formation thereof by the bendingmachining, the main magnetic flux will also be reduced.

On the contrary, in the second embodiment, no gap is generated, andalso, because the thickness of the stator yoke 31 in the radialdirection becomes larger because of the provision of the convex portions31E, the efficiency of the commutator motor can be prevented from beinglowered without causing a reduction in the main magnetic flux.

Next, a commutator motor according to a third embodiment of the presentinvention will be described with reference to FIG. 6. The commutatormotor according to the third embodiment is different from the commutatormotor 1 according to the first embodiment only in that the grooves 41 adirected to the axial direction of the stator yoke 41 are formed on theinner peripheral surface of the stator yoke 41.

As shown in FIG. 6, concave portions 41 b are defined on the innerperipheral surface of the annular body 41C that constitutes a part ofthe stator yoke 41 that retains the field magnet 12 on the innerperipheral surface. Each of the concave portions 41 b has a U-shape. Ina state where the annular bodies 41C are stacked and the field magnet 12are fixed onto the inner peripheral surface of the stator yoke 41, ahollow groove 41 a directed to the axial direction of the stator yoke 41is formed. When viewed in a cross-section of the stator yoke 41 in theradial direction, each of the concave portions 41 b is positioned in thesubstantially center of the field magnetic pole generated in the stator40 by two field magnets 12.

As a recent demand, the thickness of the stator yoke in the radialdirection is increased as described above. However, as the thicknessbecomes larger, the magnetic resistance of the stator yoke becomesmaller, and in the commutator motor 1 according to the firstembodiment, as shown in FIG. 7, an armature reaction magnetic flux B isliable to flow, which is generated due to the armature magnetomotiveforce developed by allowing a current to flow in the coil of thearmature 20, when the commutator motor is driven. Since the armaturereaction magnetic flux B impedes a flow of the main magnetic flux Adeveloped by the field magnet 12 and induces the reduction of therotational torque of the armature 20, the efficiency of the commutatormotor is lowered. As shown in FIG. 7, because the flow of the mainmagnetic flux A is divided into the right and left sides of FIG. 7through the core of the armature 20 and the stator yoke 11 of the stator10 and becomes minimum in the vicinity of the center position of thefield magnetic pole of the stator 10, the main magnetic flux A is notreduced even if the grooves 41 a are formed as in the third embodimentso that the sectional area of the stator yoke 41 at the substantiallycenter position of the field magnetic pole is reduced. In the thirdembodiment, by forming the grooves 41 a at the substantially centerposition of the field magnetic pole, the sectional area of the fieldmagnetic path at the same position of the stator yoke 41 can be reducedso that the magnetic resistance in the magnetic path along which theconventional armature reaction magnetic flux B flows as shown in FIG. 7can be increased, an influence of the armature reaction magnetic fluxcan be lessened, and the main magnetic flux is made to effectivelycontribute to the torque, thereby being capable of improving theefficiency of the commutator motor.

Next, a commutator motor according to a fourth embodiment of the presentinvention will be described with reference to FIG. 8. The commutatormotor according to the fourth embodiment is different from thecommutator motor according to the second embodiment only in that thegrooves 41 a directed to the axial direction of the stator yoke 51 areformed on the inner peripheral surface of the stator yoke 51.Accordingly, the commutator motor according to the fourth embodiment isso structured as to provide all of the features of the commutator motorsaccording to the first to third embodiments. The configuration, theposition and the number of the concave portions 41 b of the annular body51C that constitutes the groove 41 a are identical with theconfiguration, the position and the number of the concave portions 41 bof the commutator motor 1 according to the third embodiment. Also, theconfiguration, the position and the number of the convex portions 31Edisposed on the annular portion 51C are identical with theconfiguration, the position and the number of the convex portions 31E ofthe commutator motor according to the second embodiment.

Since the cross-sectional area of the field magnetic path at thesubstantially center position of the field magnetic pole can be reducedby forming the grooves 41 a, the magnetic resistance in the magneticpath along which the armature reaction magnetic flux flows can beincreased and an influence of the armature reaction magnetic flux can belessened, whereupon the main magnetic flux can be made to effectivelycontribute to the torque, thereby being capable of improving theefficiency of the commutator motor.

Further, because the field magnet 12 is retained by the convex portions31E, it is unnecessary to conduct positioning of the magnet in mountingthe field magnets 12 on the inner peripheral surface of the stator yoke51. Also, because the field magnet 12 is retained on the innerperipheral surface of the stator yoke 51 by holding the field magnet 12between a pair of convex portions 31E, it is unnecessary to fix thefield magnet 12 on the inner peripheral surface of the stator yoke 51 byusing an adhesive. Consequently, the manufacturing costs can be reduced.Also, because the thickness of the stator yoke 51 in the radialdirection becomes larger because of the provision of the convex portions31E, the efficiency of the commutator motor can be prevented from beinglowered without causing a reduction in the main magnetic flux.

The commutator motor according to the present invention is not limitedto the above-mentioned emobodiments, but various modifications areconceivable. For example, in the commutator motor according to the firstembodiment, the cross-section of the stator yoke 11 in the radialdirection is circular in both of the outer periphery and the innerperiphery thereof. Alternatively, as shown in FIG. 9, the outerperipheral positions that correspond to positions at which the fieldmagnets 12 are fixed may be linear portions 11F that are linearlynotched.

Also, in the commutator motor according to the second embodiment, twopairs of convex portions 31E radially inwardly protrude at positionscorresponding to the end portions of the field magnets 12 that areretained on the inner peripheral surface of the stator yoke 31,respectively in the peripheral direction of the stator yoke 31. However,instead of the four convex portions 31E, it is possible that parts ofthe stator yoke where the field magnet 12 is not disposed on the innerperipheral surface thereof are made larger in thickness in the radialdirection to be two convex portions 61E with which two field magnets 12are retained as shown in FIG. 10.

Also, the stator yoke 31 is circular in the outer periphery when viewedin cross-section in the radial direction. However, as shown in FIG. 11,the outer peripheral positions corresponding to positions at which thefield magnets 12 are fixed may be linear portions 31F that are linearlynotched.

Also, when viewed in cross-section in the radial direction of the statoryoke 31, as shown in FIG. 12, in order to increase the magnetic patharea of the stator yoke in the vicinity of both ends of the field magnet12, that is, in the vicinity of the magnetic poles, it is possible thatportions 31H on the outer peripheral surface of the stator yoke areconfigured by eccentric curves while providing linear portions betweenopposing convex portions 31E, 31E. In this example, each of the caulkingportions 11D may be disposed at the linear portions to which the fieldmagnet 12 is not fixed.

Alternatively, it is possible that the inner periphery is concentric andthe outer periphery is not concentric. The outer periphery is, forexample, an eccentric curve as shown in FIG. 13. In this case, each ofthe caulking portions 11D may be disposed at a position between theouter periphery and the inner periphery of a portion which is a part ofthe annular body and at which the field magnet 12 is not fixed.

Also, as shown in FIG. 14, an inner peripheral surface of the statoryoke is almost circular except a linear region at which the field magnet12 is not disposed. Even in the cases of the configurations shown inFIGS. 12 to 14, two field magnets 12 are retained on the innerperipheral surface of the stator yoke by two pairs of convex portions31E, i.e., four in total, as in the second embodiment.

Also, as shown in FIG. 15, concave/convex portions 31I are disposed atthe outer periphery of the stator yoke. The position of theconcave/convex portions 31I correspond to the center of the field magnet12. The convex/concave portions (not shown) engageable with theconcave/convex portions 31l are disposed on the housing, and theconcave/convex portions 31I of the stator yoke are engaged with theconvex/concave portions of the housing so as to fixedly position thestator yoke to the housing.

Also, a distance in the peripheral direction between the respectivepairs of convex portions 31E of the annular body is slight smaller thanthe length of the field magnet 12 in the same direction. However, thedistance may be slightly larger. In the latter case, after the fieldmagnet 12 is inserted between the convex portions 31E, the convexportions 31E are mechanically plastically deformed in a direction thatapproaches the field magnetic pole to retain the field magnet 12 betweenthe convex portions 31E.

Also, two pairs of convex portions 31E are disposed for one annular body31C that constitutes a part of the stator yoke 31 for retaining thefield magnet 12 on the inner peripheral surface thereof. Alternatively,two pairs of convex portions 31E may not be disposed for one annularbody. For example, it is possible that a pair of convex portions forretaining one of two field magnets 12 are disposed in a first annularbody and a third annular body, respectively, and a pair of convexportions for retaining the other field magnet 12 are disposed in asecond annular body and a fourth annular body, respectively. Also, it ispossible that only one convex portions is disposed on the first annularbody, and only one convex portions is disposed on the fourth annularbody to form a pair of convex portions for retaining one field magnet12.

As one preferred modification to the embodiment shown in FIG. 15,instead of two concave/convex portions 31I, only one concave/convexportion is provided at the outer peripheral surface of the stator yoke,and grooves corresponding to the grooves 41 a as shown in FIGS. 6 and 8are formed on the inner peripheral surface of the stator yoke.

Also, in the commutator motor according to the third embodiment, theconcave portion 41 b is formed with a configuration that is notched in asubstantially U-shape toward the exterior of the annular body 41C.However, the shape is not limited to this configuration. For example,the concave portion 41 b may have a triangular shape or a substantiallysemicircular shape.

Also, the grooves 41 a are hollow in the above embodiments. However, thestacked annular bodies 41C may be fixedly adhered to each other byfilling an adhesive into the hollow space. The filling of the adhesiveis conducted in such a manner that after the annular bodies 41C arestacked on each other and the field magnet 12 is fixed on the innerperipheral surface of the stator yoke 41, a tube for filling theadhesive is inserted into one open end of the groove 41 a, and theadhesive is injected into the groove 41 a. A large vibration may bestructurally applied to the commutator motor portion depending on thekind of a power tool. In this case, when the adhesive is filled in thegroove 41 a, and the respective annular bodies 41C are adhered to eachother, the respective annular bodies that are fixed by caulking andstacked on each other at the caulking portion 11D can be prevented frombeing detached from each other. The filling of the adhesive may also beconducted in the fourth embodiment.

Also, in the commutator motor according to the fourth embodiment, thegrooves 41 a are formed. However, as shown in FIG. 16, the grooves 41 amay be replaced by non-magnetic bodies 51J. That is, when viewed incross-section in the radial direction of the stator yoke, asubstantially center portion of the stator yoke at a position where thefield magnet 12 is fixed may be formed by the non-magnetic body 51J. Inthis case, the stator yoke is not structured by stacking the annularbodies on each other, but structured in such a manner that a pluralityof iron plate-like bodies each of which formed in two substantiallysemi-circular and substantially the same configuration which form a partof virtual annular bodies are stacked on each other, and bar-likenon-magnetic bodies 51J that extend in the axial direction of the statoryoke are connected between end portions of the stacked plate-likebodies, and the stator yoke becomes annular in a cross-section takenalong a face perpendicular to the radial direction of the stator yoke.Also, it is possible that each of the annular bodies is made up of twosubstantially semicircular iron plate-like bodies that form a part ofthe annular body and two substantially arc-shaped plate-likenon-magnetic bodies that connect both ends of the two substantiallysemi-circular portions, respectively, and the annular bodies are stackedon each other to structure the stator yoke.

Also, in all of the above-mentioned embodiments, the caulking portion11D is structured in such a manner that one surface side of theplate-like annular body is completely convex portions and the othersurface side is completely concave portions. Alternatively, the convexportions and the concave portions may be mixed together on one surfaceside or the other surface side.

The housing and the stator yoke are structured as separate components inthe above-mentioned embodiments, however, the stator yoke may serve asthe housing.

The stator yoke can have rectangular or polygonal outer configuration.With this structure, the sectional area of the yoke portion becomeslarge, thereby making it difficult to have the permanent magnetmagnetically saturated.

Further, the battery is used as the power source in the above-mentionedembodiments. However, other type of power source is available.

The foregoing description of the preferred embodiments of the inventionhas been presented for purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and modifications and variations are possible in lightof the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and its practical application to enableone skilled in the art to utilize the invention in various embodimentsand with various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto, and their equivalents.

1. A commutator motor, comprising: a stator comprising a stator yokehaving a tubular shape and extending in an axial direction and having aninner peripheral surface, and a field magnet fixed to the innerperipheral surface of the stator yoke for providing a field magneticpole in the stator; and an armature rotatably disposed within thestator; wherein the stator yoke is comprising at least one of (a) aplurality of plate-like annular bodies having iron parts which arestacked on each other coaxially in the axial direction of the statoryoke, and (b) a plurality of substantially identically configured ironplate-like arcuate bodies disposed at such position as to form a part ofvirtual annular bodies and which are stacked on each other coaxially inthe axial direction of the stator yoke; wherein the plurality ofplate-like annular bodies or the plate-like arcuate bodies adjacent toeach other in the stacked direction are fixedly connected to each otherby caulking, wherein a non-magnetic portion is defined by a hollowgroove extending in the axial direction of the stator yoke and formed inthe inner peripheral surface of the stator yoke with a depth of thegroove extending partially into the stator yoke, and wherein the statoryoke has the non-magnetic portion at a substantially center portion ofthe field magnetic pole in a radial cross-section of the stator yoke. 2.The commutator motor as claimed in claim 1, wherein the grooves areU-shape, triangular shape or a semi-circular shape.
 3. The commutatormotor as claimed in claim 1, wherein adhesive is formed in the grooves.4. The commutator motor of claim 1, wherein the plurality of plate-likeannular bodies or the plate-like arcuate bodies each have a concaveportion and are fixedly connected to each other by caulking the concaveportion and mating to a convex portion of an adjacent one of theplurality of plate-like annular bodies or the plate-like arcuate bodies.5. A commutator motor, comprising: a stator comprising a stator yokehaving a tubular shape and extending in an axial direction and having aninner peripheral surface, and a field magnet fixed to the innerperipheral surface of the stator yoke for providing a field magneticpole in the stator; and an armature rotatably disposed within thestator; wherein the stator yoke is constituted by one of (a) a pluralityof plate-like annular bodies having iron parts which are stacked on eachother coaxially in the axial direction of the stator yoke, and (b) aplurality of substantially identically configured iron plate-likearcuate bodies disposed at such position as to form a part of virtualannular bodies and which are stacked on each other coaxially in theaxial direction of the stator yoke; wherein the plurality of plate-likeannular bodies or the plate-like arcuate bodies adjacent to each otherin the stacked direction are fixedly connected to each other bycaulking, wherein the stator yoke has a non-magnetic portion at asubstantially center portion of the field magnetic pole in a radialcross-section of the stator yoke, and wherein the stator yoke isconstituted by a plurality of substantially identically configured ironplate-like arcuate bodies, and wherein the non-magnetic portion is madefrom a non-magnetic bar members extending in the axial direction of thestator yoke and held between confronting ends of the arcuate bodies, acombination of two arcuate bodies, and two non-magnetic bar membersproviding an annular body.